Liquid crystals and electrochromic substances are known as materials that change their optical transmittance values in response to an electric field applied thereto. Liquid crystals have already been widely used in displays. Electrochromic substances are substances that exhibit chromism (reversible color change) through an electrochemical oxidation-reduction reaction, and can be considered as finding various applications including in a paper-like display (so-called “electronic paper”). Materials of this kind, which are sensitive to electric fields, are advantageous in that they can be more easily controlled than substances that are sensitive to light or heat, in terms of the configuration of their control means.
Although many electrochromic substances are high molecular weight compounds that are chemically synthesized, JP 2000-502398W discloses that tin oxide doped with antimony or niobium exhibits electrochromism. This publication reports that a cyclic color change following an electric signal of 50 mHz occurred (the response speed in this case is 20 seconds). The above-mentioned publication also describes that mixing a light-colored pigment such as titanium dioxide, silica, or alumina with doped tin oxide is effective in improving contrast.    Patent Document 1: JP 2000-502398W